The above-mentioned measuring system typically comprises at least a sensor comprising at least one sensing element placed in a flow of medium, so that the electric charge of a moving particle induces an electric signal in the sensing element, and means comprising a processor unit with a signal-processing unit connected to the sensor to collect the signals from the sensing element.
One adaptable measuring system has been introduced in the U.S. Pat. No. 6,031,378 whose entire contents are hereby incorporated by reference. When the above measuring system is used for velocity measurements, it comprises at least two sensing elements placed at a known distance from each other in the flow direction of medium, arranged so that the electric charge of a moving particle induces an electric signal in the sensing elements. The velocity measurement is based on detecting a phase angle between the signals of these sensing elements. In addition to motion in the direction of the flow, particles typically exhibit cyclic motion in a direction substantially perpendicular to the flow. Due to a physical distance in the flow direction between these two sensing elements, the cyclic motion causes a phase angle between the signals of these sensing elements. The measuring system comprises means for calculating the phase angle between collected signals and means for calculating the particle velocity according to calculated phase angle.
A device for contactless measurement of a state variable of a flowing medium containing electrically charged particles has been suggested in a patent application WO 98/26255. A two dimensional sensor element is arranged parallel to the particle trajectory or at least two sensor elements, for example electrodes, are arranged in series seen in the flow direction. Charges on these electrodes are influenced by electrically charged particles flying past the electrodes, and voltage signals are produced using suitable amplifiers. A voltage change component is evaluated to determine particle concentration. In order to determine particle velocity, the transit time is evaluated. In order to determine the particle throughput, both the signal strength and the time shift of the individual signals are comparatively evaluated.
Particles do not necessarily follow a straight path in the z direction, direction of the flow (see FIG. 1). The particles also have in addition to the cyclic motion, other sideways motion components in the x and y directions, perpendicular to the flow, due to gas turbulence and collision between themselves. The particle velocity differs from the gas velocity due to a slip between the gas flow and the particle flow. The gas has a higher velocity than the particles. It has been observed that the slip is not constant. The slip depends on the concentration of particles in the gas and, naturally on the particle size. For instance heavy roping of the coal dust in a burner duct leads to an increased local concentration of the particles and therefore to a bigger slip.
The movement of a particle in the flow direction induces a certain, quite slowly varying signal component to a measuring electrode. A cyclic movement of a particle in the direction perpendicular to the flow direction induces an alternating signal component to the measuring electrode. In principle each charged particle thus causes a signal component having a slowly varying part and an alternating part. A signal measured using an electrode is a sum of the signal components relating to various charged particles, and it typically comprises a slowly varying DC—(direct current) part and an AC—(alternating current) part. The DC-part of a measured signal typically originates from the flow (in other words, from the movement of particles in the direction of the flow). The AC part of a measured signal is due to, for example, a cyclic motion of the particles, which motion is perpendicular to the flow direction and is caused by a fan, gas pressure gradients, non-uniform charge distribution of the particles and/or in non-uniform velocity distribution of the particles.
Particles have a fairly strong sideways motion due to gas turbulence and collisions between themselves. This sideways motion will create heavy disturbances to the above mentioned velocity measurement, which uses the cyclic motion. The velocity measurement of this measurement system is disturbed because the signals created on the down and upstream sensors do have a phase angle, which is related to various other factors in addition to the particle velocity along the conduit. Therefore, above measuring system seems to be highly sensitive to interference from these sideway motions.
Another prior art method and apparatus for particle analysis is described in the U.S. Pat. No. 5,296,910. The disclosed method comprises the steps of supplying particles to be tested to a sensing volume of a laser Doppler velocimeter, exciting the particles in the sensing volume with a plurality of forces which are orthogonal over an interval corresponding to a cycle of a fundamental frequency of said forces and which have a zero mean force over said interval, and sensing resulting motion of the particles in the sensing volume to obtain a sensor signal, wherein said sensor signal includes components representative of physical characteristics of the particles. The above method suffers the disadvantage that light-scattering instruments used in the method are vulnerable to get dirty in process conditions raising inaccuracy in the measurement. Further, opaque samples cannot be measured.
A method for measurement of the mass of a charged particle is described in the U.S. Pat. No. 4,010,366. The disclosed method involves ejecting the particle into a sampling device made up of a tube comprising a Faraday cage with a region of a grounded conductive material on either end of it. The particle flows through the tube in a stream of gas and as it passes through the Faraday cage it induces a charge on the cage wall. By measuring the magnitude of the induced charge or its duration in the cage, the magnitude of the charge on the particle or the mass of the particle can be determined. Because this prior art method requires sampling prior to measuring, the method is unsuited to on-line measurement of particle size.
It is an object of the present invention to provide a new, reliable and, as to its design, a simple measuring system and a method for measuring particle velocity and/or particle velocity distribution and/or particle size and/or particle size distribution in e.g. flowing powdery mediums, gas flows, or material webs in various kinds of processes and environments, in order to obtain measuring values which can be used for controlling these processes or which can be used to evaluate their conditions.
It is a particular object of this invention is to provide a measuring system and a method which eliminates the impact of the inherent disturbances on the particle velocity measurement.
Another object of this invention to provide a measuring system and a method which is also suited for use in industrial processes under severe, e.g. dust-laden and/or interference laden conditions.
It is a further object of the invention to use the method in controlling of a burner system or a mill in order to constitute a certain particle velocity and/or particle size and/or a minimum magnitude of fluctuations in the burner system or the mill, or in controlling of particle size or particle velocity e.g. in a grinding circuit or in a transport duct, or in controlling of rate of air flow and fuel flow in a diesel motor.
It is a further object of the invention to provide a measuring system, which does not require calibration before or while measuring particle velocity.
It is a further object of the invention to provide computer programs, which calculates particle velocity and/or particle size from collected signals within relevant frequency band.
The above stated objects are achieved by means of measuring systems, methods, computer programs and uses which are characterized by what have been stated in the characterizing part of the appended independent claims.
A measuring system for measuring properties of particles in a medium flow according to the invention comprises typically means for filtering collected signals arranged to pass signals at a relevant frequency band or at relevant frequency bands so that measurements can be made in a relevant frequency band or frequency bands. In order to carry out measurements calculations are carried out by the signal-processing unit connected to the sensor. The signal-processing unit can be a computer or a device based on a simple microprocessor. The means for filtering collected signals, here also called filter, will then have a filtering effect on the measuring signals, which are conducted from the sensor to the signal-processing unit. Thus, only signals in the relevant frequency band or frequency bands are used in measuring the properties of particles. Filtering of the measuring signals results in the fact, that the measurements will be made on the frequency band or bands of the pressure gradients affecting the particle motion. Interference at known frequencies can, if necessary, be filtered off. A typical example of an external interference, which is preferably filtered off, is the 50/60 Hz frequency of the electrical network.
The relevant frequency band or bands are preferably defined by frequencies which exist in consequence of one or more process equipment or a process occurrence affecting to the medium flow. The relevant frequency band may comprise one or more frequencies and, thus even a single frequency is called as frequency band in this connection.
The medium flow referred to above can be gas flow or fluid flow or a two-phase flow stream including liquid and gas components. The particle can advantageously be powdery particle, pulverous particle, pulverulent particle, granule particle, ionized gas or liquid molecules.